Effect of chlorine on cultivability of Shiga toxin producing Escherichia coli (STEC) and β-lactamase genes carrying E. coli and Pseudomonas aeruginosa.

The worldwide spread of toxin-producing and multi-drug resistant bacteria in water, food and the environment is considered a major threat to human health. Drinking water quality is controlled by inspection of fecal indicators presence whereby viable contaminants will be efficiently reduced by chlorination which is a common process for disinfection. However, the all-out efficiency is arguable, because bacterial regrowth has been documented after disinfection. In this study, we investigated the stability of Shiga toxin producing Escherichia coli (STEC) and β-lactamase expressing E. coli and Pseudomonas aeruginosa isolates, both equipped with multiple or single β-lactamase resistance genes. The aim of the study was to analyze the efficiency of chlorine (Cl) disinfection against shigatoxigenic or β-lactamase producing bacteria. Cl reacts with the bacterial cells after first contact. Counts of antibiotic resistant E. coli were lower after short than upon extended Cl treatment. P. aeruginosa counts decreased moderately upon 15-60 min treatment with 1.2 mg Cl/l, while cells adapted to tap water were not cultivable anymore. We assume that the bacterial physiology changed to a temporary non-cultivatable state at first Cl contact followed by resuscitation of some cells at later stages. STEC viability went down continuously at low Cl concentrations and these toxigenic E. coli isolates exhibited slightly increased stability to Cl treatment compared with non-toxigenic E. coli. Controlling the efficiency of disinfection, realistic counts of cultivatable cells are achieved after extended Cl action.